In recent years, accompanying with rapid progress of an information-oriented society, techniques regarding information-related apparatuses have rapidly been progressed and spread. Of these, a display device has been used for television, for a personal computer, for an information display at a station, airport, etc., and for providing other various kinds of information. In particular, plasma display is attracted attention in recent years.
In such an information-oriented society, it has been worried about influence of an electromagnetic wave emitted by these display device. For example, it has been worried about influence on neighbouring electronic devices or influence on a human body. In particular, influence of an electromagnetic wave which exerts to health of a human body cannot ignore, and a strength of electromagnetic field emitted to the human body is required to be reduced. To comply with these demands, various transparent conductive materials to be used for an electromagnetic wave shielding material are developed, and there are disclosed in, for example, JP 9-53030A, JP 11-126024A, JP 2000-294980A, JP 2000-357414A, JP 2000-329934A, JP 2001-38843A, JP 2001-47549A, JP 2001-51610A, JP 2001-57110A, JP 200160416A, etc.
As processes for preparing these transparent conductive materials, there has generally been used a process in which a conductive metal such as silver, copper, nickel, indium, etc. is formed on a transparent resin film as a metal thin film by the method such as a sputtering method, ion plating method, ion beam assist method, vacuum deposition method, wet coating method, etc. However, according to these conventional methods, manufacturing processes become extremely complicated, so that they cause problems of high cost and poor productivity.
Also, as the other characteristics required to the transparent conductive material, there are conductivity and light transmittance. To heighten conductivity, it is necessary to prepare a metal thin film fine pattern having a width and thickness with a certain degree, but when a line width of a pattern comprising a metal which shields light is made thick, then a transmittance lowers. To comply with the both requirements simultaneously, it is necessary to prepare a fine metal pattern having sufficient conductivity, in particular, uniform pattern with a minimum width is required to be prepared, but this cannot be satisfied by the conventional methods.
For preparing a uniform pattern, in recent years, there has been proposed a method of using a silver salt light-sensitive photographic material containing a silver halide emulsion layer(s) as a precursor of the transparent conductive material. For example, in WO 01/51276A (Patent Literature 1) or JP 2004-221564A (Patent Literature 2), there have been proposed a process for preparing a transparent conductive material by pattern-wisely exposing a silver salt light-sensitive photographic material, subjecting to development, and then, subjecting to a metal plating treatment. As a method of using a silver salt light-sensitive material similarly, there has been proposed a method of using a silver salt diffusion transfer method, and there is, for example, JP 2003-77350A (Patent Literature 3), etc.
As a precursor for the conductive material, when a higher conductance is required in a conductive material using a silver salt light-sensitive photographic material, a metal plating treatment is carried out. However, when an electrolytic metal plating is to be applied to a particularly fine metal pattern, in the fine pattern, resistance of the pattern becomes too high for applying an electro-lytic plating to the fine pattern, whereby only a portion to which electricity is applied is occasionally plated. Accordingly, to carry out the electrolytic metal plating treatment uniformly, a process for preparing a conductive material, which gives high conductivity even in a fine metal pattern, has been required to be developed.
Moreover, in a conductive material using various kinds of silver salt light-sensitive photographic material as a precursor for the conductive material, conductivity is sometimes fluctuated during the preservation. As an example in which the above causes a problem, there is a ¼λ type electromagnetic absorber. In the ¼λ type electromagnetic absorber, it employs a structure in which a transparent resistance film which is equivalent to 377Ω/□ that is a characteristic impedance of a free space is provided at the position apart from ¼ of the wavelength λ of the electric wave absorbed from an electric wave reflecting material. In a conductive material using a silver salt light-sensitive photographic material as a precursor of the conductive material, a resistant material having high transparency can be prepared, but due to the problem of fluctuation of conductivity, the resistivity is deviated and the characteristics of the electromagnetic absorber is deteriorated during its preservation. As can be seen from this example, it has been required to provide a process for preparing a conductive material which has not only sufficiently high conductivity, but also high storage stability simultaneously.
The conductive material is further used in various fields such as shielding of infrared rays of construction materials or automobiles, antistatic materials for electronic devices or cell phones, heat rays of frosted glass, a wiring of circuit boards or IC cards, a coating for providing conductivity to resins, through hole, circuit itself, etc.
In particular, in preparation processes of a semiconductor device, liquid crystal display device, plasma display device, etc., a conductive metal thin film is sometimes to be formed to form an electrode or wiring, etc. In these preparation processes, the metal thin film is generally prepared by a thin film-forming method such as a vacuum deposition, sputtering, CVD method, etc. Such a thin film-forming method is generally carried out under an atmosphere of reduced pressure, so that a substrate, etc. is required to be provided in a reaction vessel such as a vacuum chamber, etc. Accordingly, an object to be formed by a metal thin film is larger, a vacuum chamber which can contain such a large sized object therein is necessary, whereby there is a problem that preparation of such a material is difficult.
Also, in the preparation of a flexible printed circuits, for forming a conductive part at a concave portion such as a contact hole or a via hole, etc., there is a case of using, for example, silver paste in which silver particles having a size of 1 μm or so are mixed and kneaded in a resin, but conductivity is ensured by contact of silver particles with each other, so that there is a problem that conductivity is low.
In JP 3-281783A (Patent Literature 4), there is disclosed a method in which paste having been dispersed metal ultra fine particles therein is coated, and after coating, this is sintered at a temperature of about 900° C. to form a metal thin film. According to this method, a metal thin film can be formed by coating paste, so that it is suitable for forming a metal thin film with a large area or forming a metal thin film on a support having a complicated surface shape.
Also, in JP 2001-35255A (Patent Literature 5), there is disclosed a ultra fine silver particles-independently dispersed liquid which comprises an organic solvent which evaporates at a drying-calcinating step for forming a silver wiring on a semiconductor substrate, and silver-containing ultra fine particles having a particle size of 0.01 μm or less being mixed, each surface of said ultra fine particles being covered by said organic solvent to be independently dispersed, a viscosity at room temperature being 50 mPa·s or lower. Thus, an extremely high conductivity can be obtained by calcinating the material at a temperature of 300° C. to give a silver thin film.
However, the metal colloid solution in which these metal ultra fine particles are dispersed in a liquid is prepared by evaporating a metal under reduced atmosphere, and monodispersibility is high, so that it is suitable for, for example, a nozzle type pattern forming device such as an ink-jet system, etc. However, for obtaining conductivity, a temperature of at least 200° C. is necessary, so that a usable substrate is limited to a heat-resistant support such as glass, polyimide, ceramic, etc.
On the other hand, it has been known a process for preparing metal ultra fine particles coated by a dispersant by reducing an aqueous silver nitrate solution, etc. with a reducing agent in the presence of a dispersant which is the so-called protective colloid.
For example, in American Journal of Science, Vol. 37, P476-491, 1889, M Carey Lea., there has been reported a method of obtaining a metal colloid solution which comprises citric acid or a salt thereof is added to an aqueous solution of a metal salt as a dispersant, a reducing agent such as a primary iron ion, etc. is then added to the same, and then, desalting and condensation are carried out. Also, in Experiments in Colloid Chemistry, 1940, p. 19, Hauser, E. A. and Lynn, J. E., there has been reported a method of obtaining a metal colloid solution using dextrin as a dispersant and as a reducing agent. However, even when a pattern formed by using these metal colloid solutions is sintered, a dispersant is difficultly evaporated so that the resulting conductivity was low.
Also, as metal ultra fine particles reduced in an aqueous solution and showing good conductivity, for example, there are disclosed, in JP 2002-338850A (Patent Literature 6), a metal colloid solution and a process for preparing the same which is prepared by using a polymerized material of a (thio)phenol derivatives) and to give a conductive film by calcinating under mild conditions. However, it was a metal colloid solution showing a high resistance value by calcinating at lower than 100° C. In JP 2005-081501A (Patent Literature 7), there are disclosed stable metal ultra fine particles and a process for pre-paring the same which can establish practically employable conductivity at a low temperature sintering. There is disclosed that practically employable conductivity can be established by calcination at 140 to 220° C., but the process yet requires the calcination step.
These metal ultra fine particles contained in the metal colloid solution are generally coated by a dispersant, so that in the state that a dispersing medium such as water, etc., has been evaporated by a drying step, connection between said metal ultra fine particles is not formed, whereby the resulting material does not show any conductivity, or extremely low even when it showed conductivity. Thus, to obtain good conductivity which can be used as a conductive material, it is necessary to carry out a calcination step to decompose and evaporate said dispersant, and to form mutual connection of said metal ultra fine particles due to fusion thereof, whereby not only an energy therefor is necessary but also a usable substrate is limited in the point of heat-resistance.
In order to develop conductivity without heating, or by low temperature heating, there has been carried out a device on a substrate. For example, in JP 2004-127851A (Patent Literature 8), there is disclosed a conductive film-complex material wherein a conductive film-complex material comprises laminated conductive films formed by drying metal colloid, and using an image receiving layer containing at least a porous inorganic filler, and in JP 2005-32458A (Patent Literature 9), there is disclosed a conductive film-complex material wherein the conductive film-complex material comprises laminated conductive films formed by drying a metal colloid solution on a substrate, and an intermediate layer comprising a water-soluble resin or hydrophilic resin and having a ten-point average surface roughness Rz according to JIS B 0601 is 3 μm or less is interposed between the conductive film and the substrate, and volume resistivity thereof is 10×10−5 Ω·cm or less.    Patent Literature 1: WO 01/51276    Patent Literature 2: JP 2004-221564A    Patent Literature 3: JP 2003-77350A    Patent Literature 4: JP 3-281783A    Patent Literature 5: JP 2001-35255A    Patent Literature 6: JP 2002-338850A    Patent Literature 7: JP 2005-081501A    Patent Literature 8: JP 2004-127851A    Patent Literature 9: JP 2005-32458A